The Sexually Dimorphic Nucleus of the Preoptic Area (SDN-POA) of the rat hypothalamus is a morphological marker of the process of sexual differentiation of the brain. The differentiation of this nucleus (in terms of volume) takes place during the neonatal life of the animal. In addition, this process is influenced by the gonadal steroid environment during the early postnatal period. 30% of the presumptive SND-POA neurons can be specifically identified during development by administering tritiated thymidine on day 18 of gestation to the pregnant female. We have utilized this fact in order to determine the migratory pattern of immature neurons as they form the SDN-POA during the first ten days of postnatal life. In the proposed studies, the ability to specifically label presumptive SDN-POA neurons will allow the SDN-POA to be used as a model system in order to continue to investigate the interaction of the gonadal steroid environment of the perinatal animal with the morphological development of the central nervous system. Thymidine autoradiography, light and electron microscopy, and catecholamine histofluorescence will be used in order to investigate the following questions. 1) What is the morphology of SDN-POA neurons and the surrounding neuropil? 2) Do gonadal steroids influence SDN-POA neuronal morphology during differentiation by affecting neurogenesis, neuronal migration, neuronal death and/or neuronal organelle formation? 3) What is the time course of development of the sexual dimorphism of catecholamine varicosities in the SDN-POA? 4) Can the short-tailed opossum be used as a model animal in order to describe the development of the SDN-POA during the 'prenatal period'? The results of this research program will help explain the mechanisms by which the hormonal environment influences a) neuronal growth and development and b) the sexual characteristics of an individual.